Improved referral indication in office

This small patch is just an improvment to encourage players to invite their friends to the game. The icon makes the message more attractive.

The second part (maybe not that important) is a donation message in case you want to recive some help to keep your server up. This will require you to manually "Entschädigung zahlen" after reciving the donation.

Here are the icons since i have no idea how to upload them :P  
![invite](https://cloud.githubusercontent.com/assets/6611118/3347511/7de21d14-f8e5-11e3-99e9-1c4d4f913cd7.png) invite.png
![bonus](https://cloud.githubusercontent.com/assets/6611118/3347512/8325d98c-f8e5-11e3-8124-d1dbf5045f74.png) bonus.png
![notas2](https://cloud.githubusercontent.com/assets/6611118/3347471/82933c1e-f8e3-11e3-99bd-644a92b69590.png)